Abstract This proposal requests partial support for the FASEB Science Research Conference on Dynamic DNA Structures in Biology, which will be held June 7-12, 2020, at Oak Island, Nova Scotia. The 2020 conference will be the fifth meeting on Dynamic DNA Structures in Biology, a highly interactive meeting that brings together a diverse, international community of researchers who investigate mutational processes caused by unusual DNA structures. After its initial discovery, DNA was believed to be a rigid, right-handed double helix. It is now appreciated, however, that DNA is highly dynamic molecule and can assume an enormous variety of conformations such as hairpins, cruciforms, left-handed helices, three- and four-stranded helices, G- quadruplexes and slip-stranded configurations. Such structural transitions are promoted by the separation of DNA strands, and hence are associated with the processes of replication, transcription and recombination. Formation of many of these structures expose chemically reactive nitrogenous bases in single-stranded DNA, rendering them hypersensitive to DNA damaging agents. They thus represent hotspots for mutagenesis by environmental factors including oxidizing agents and sulfites. Structure-prone DNA sequences are central to the normal functioning of the genome, for example, transitions in DNA structure are important for regulating the activation of transcription and the specialized recombination responsible for immunoglobulin class switching. Unscheduled transitions, however, can have pathological consequences. Of particular significance, the expansion of structure-prone repeats underlies more than thirty hereditary neurological and developmental diseases in humans. The long-term objective of this Conference is to enhance our understanding of how dynamic DNA structures form, how they are resolved, how they contribute to normal genetic processes and how they promote pathological outcomes such as genetic disease, cancer and aging. Our aims are to explore current understanding of DNA structural transitions, to define novel mechanisms that promote formation and resolution of dynamic DNA structures, to stimulate collaborations, and to foster the long-term development of this important area by promoting participation of junior scientists. To that end, we will convene ~100 participants for five intense and highly interactive days of science. The program will include two opening keynote talks, eight morning/afternoon scientific sessions and informal evening poster sessions. New this year will be sessions on ?RNA-mediated genome instability? and ?Single-molecule analysis of replication and repair structures?. Another new feature will be to involve trainees and early career scientists in the planning and content of ?meet the experts? and career workshop discussions at the conference. One third of the talks will be chosen from submitted abstracts, with preference given to trainees and junior investigators. The significance and strength of this Conference is that it uniquely brings together investigators with a common focus on dynamic DNA transitions, but who have highly varied backgrounds, expertise and experimental approaches.